The invention relates to solid state electronic power switching circuitry and more particularly to switching circuitry for controlling the power delivered to a load such as a ceiling fan or light.
Controllable power switching systems are known employing triacs wherein a control circuit connected to a gate of the triac determines the point in time during each supply voltage cycle when the device will conduct. By changing the conduction point, the power to the load can be varied.
With such prior art systems, excessive mechanical and electrical noise occurs. When the triac fires, the instantaneous power (kick) can cause laminations of electric motors such as on ceiling fans to vibrate with a result that a 60 cycle hum eminates from the fan motor. Even with electric light bulbs, a distinct audible hum can be detected which can be quite annoying to the user. In addition, the current surges cause radio frequency interference to be transmitted through the power lines.
Although power to motors or lights can be controlled by use of line chokes such as saturable reators and magnetic amplifiers which do not have the electrical or mechanical noise problems, such devices dissipated excessive heat due to internal resistive losses in the choke windings. Also, such units are difficult to switch because of the resulting arcing.
Another disadvantage of prior art electronic switching systems is the use of mechanical switches such as rotary switches whose contacts may deteriorate resulting in a short service life.